Wraiths are a weird phenomenon, rare enough that the majority of the Mythos has no idea they exist.
For the record, they are not ghosts, which are members of the Dream. Wraiths are pieces of most of the Seven Peoples (apart from members of the Sun) who have died and who can never be whole, for most of their essential selves has been destroyed.
A white thing—a puff, a phantom, a see-through blobby shape like a pillowcase underwater—flew through the wall and between them, bringing with it a cold wind and wearing an impossibly solid red bowtie near its top. […] It had a shape, of sorts—curvy and blobby and really not humanoid. There were no limbs. The red bow-tie indicated the possibility of a head, but there was no face; above the tie, the blob tapered, ending in a sort of empty-windsock sag.
People of the Sun who died and left pieces behind are called Slivers.